The subject matter of the present invention relates generally to veneer lathe apparatus for peeling sheets of veneer from a rotating log by means of a lathe knife of adjustable pitch and, in particular, to an electrical control system for automatic adjustment of lathe knife pitch angle in response to reduction of the diameter of the log during peeling.
As shown in U.S. Pat. No. 4,708,180 of R. A. Browning, et al., issued Nov. 24, 1987, it has heretofore been the practice to provide a veneer lathe with automatic adjustment of the lathe knife pitch angle by a mechanical control including a cam follower roller on the knife carriage which rolls along a pitch rail cam member. The slope of the pitch rail is set manually or by other mechanical adjustment means prior to the start of peeling and the knife pitch is then adjusted entirely by movement of the cam follower roller along the surface of the pitch rail during peeling. In most cases, the pitch rail is linear with a constant slope. However, such roll may be provided with a custom profiled end portion of changing slope for small diameter logs as discussed in the Browning patent. Unfortunately, such a custom-made pitch rail having an end portion of different slope does not apply to all species or types of wood. In addition, manual adjustment of the pitch rail to set the slope angle at the start of the peel is time consuming and inaccurate. The above-cited Browning patent also discloses the use of an automatic control means employing a computer for adjusting the gap between the lathe knife and a nose bar roll engaging the log immediately in front of the knife in response to changes in the log diameter and knife pitch during peeling. However, there is nothing in this patent which teaches that the pitch angle of the knife should be adjusted automatically by an electrical control system including a computer in accordance with knife pitch profile tables stored in the memory of such computer in the manner of the present invention.
During the veneer peeling process, better quality veneer is usually produced when the lathe knife pitch angle is varied as the radius of the log diminishes during peeling. Typically, the knife pitch angle, which is the angle the front cutting edge of the knife makes with a vertical plane passing through the tip of such knife, varies from a positive angle or "lead" where the knife is tipped away from the log for logs of large diameter to a negative pitch angle or "heel" where the knife blade is inclined toward the log for small diameter log blocks. The difficulty is that the manner in which the knife pitch angle should be varied during peeling is different for different species or types of wood. This requires frequent manual adjustment of the slope of the pitch rail and replacement of any custom profile pitch rail with another profile when such rail is provided with an end portion of changing slope. These problems are overcome in the present invention by electrical control of the lathe knife pitch angle using a digital computer having digital data representing different profiles of pitch angle adjustment provided as a plurality of different knife pitch tables stored in the memory of the computer.
In the electrical control system of the present invention, the computer is employed to control the knife pitch by adjustment of the vertical position or height of the cam follower roller on the knife carriage to support such carriage at different heights above the pitch rail. In the preferred embodiment, the height adjustment of the cam follower roller is accomplished by means of an eccentric which is rotated by a lever arm connected to the piston rod of a pitch adjustment cylinder operated by a servo valve in response to the pitch control output signal of the computer. The cylinder contains a transducer which produces a piston rod position signal corresponding to such height which is fed back to the computer to provide a closed loop feedback control in order to accurately set the pitch angle within 0.01 degree as a function of the horizontal position of the main knife carriage along the pitch rail during peeling.
The present invention has the advantage that different pitch angle profiles for adjustment of the lathe knife pitch angle are easily defined and revised by changing the data entries stored in the computer memory. In addition, the computer enables quick selection of the appropriate pitch profile from a plurality of pitch profile tables stored in such memory. Also, the ability to develop more optimum pitch profiles for different wood species and different wood conditions is made possible. As a result, the quality of veneer is improved, and the production output of the veneer lathe is increased.